The demand for light portable high performance electric devices, such as video cameras, mobile phones, and laptop computers has increased. Accordingly, much research has been conducted in order to develop batteries with enhanced cycle and capacity characteristics to be used as the driving source. In particular, rechargeable lithium batteries having three times the energy density per unit weight as conventional lead storage batteries, nickel-cadmium batteries, nickel-hydro batteries and nickel-zinc batteries, and batteries having a shorter recharging time have been developed.
In general, an electrochemical cell includes a cathode, an anode, and an electrolyte. During cell discharging, oxidation occurs at the anode and reduction occurs at the cathode. Electric charges created during the oxidation-reduction reactions migrate via the electrolyte. The electrolyte blocks the migration of electrons from the anode to the cathode or from the cathode to the anode by cutting off electric contact between the cathode and the anode. An electrolyte that is commonly used includes a multi-porous separator, electrolytic solutions and salts, and in particular, a lithium salt.
Many different solutions have been proposed for the protection of lithium anodes including coating the lithium anode with interfacial or protective layers formed from polymers, ceramics, or glass. The interfacial or protective layers, however, must be able to conduct lithium ions. For example, anode materials using a single metal (or alloy) layer in a lithium battery in which oxidation reaction occurs are disclosed in U.S. Patent Publication Nos. 2000-713997 and 2002-0182508, and U.S. Pat. No. 6,537,701. In addition, U.S. Patent Publication No. 2002-0012846 discloses the use of a single metal layer or an alloy layer as an intermediate protective layer, which is added to a multi-layer surface protective film composed of an inorganic electrically conductive material, a polymer and a metal layer.
In contrast, U.S. Patent Publication No. 2002-0012846, discloses a protective layer formed only of a metal, which is capable of forming an alloy with lithium when a vacuum deposition method is used. Although the lithium alloy is less reactive than pure lithium, it still strongly reacts with the components of the electrolyte. As a direct result, the surface of the protected anode becomes very rough, which is undesirable. In particular, less corrosion of the outer or inner metal (or alloy) layer occurs when the layer has a smooth surface than when it has a rough surface. Also, the upper inorganic layer or the polymer layer can have stronger adhesion to a smooth surface than to a rough surface.
When a metal that does not form an alloy with lithium, such as Cu and Ni, is used, a surface coating having a very smooth surface can be formed. However, if this coating film has a thickness greater than about 1000-2000 Å lithium diffusion may be prevented. Also, the lithium anode with the coating film is electrochemically less activated. Furthermore, when an external polymer layer is formed, a specific solution contacts the electrode surface. Thus, a surface film composed of a metal capable of forming an alloy with lithium does not perform as well as a protective layer for preventing corrosion when the surface film contacts a specific solution, because the concentration of the lithium in the lithium alloy is high.
Alternatively, Cu, Ni and Fe films composed of non-alloy metals with thicknesses of greater than 2000 Å are capable of protecting the lithium, but may cause deterioration of the cycling characteristics of the anode. Therefore, it may be necessary to form a temporal protective layer at the lithium surface. This layer must positively influence the anode storage and cycling behavior, and/or it must protect the lithium from corrosion during the process of forming the subsequent polymer layers. This layer must not hamper lithium dissolution and deposition during the cycling. It is desirable to have a smooth surface for such an intermediate protective layer because it has high protective properties and guarantees a good adhesion with the subsequent polymer layer or inorganic layer.
Therefore, a temporary protective layer may be formed on the lithium surface and the protective layer. This layer, however, must improve the capacity characteristics and cycling characteristics of the anode, or suppress corrosion of the lithium surface when the polymer layer is formed. However, in a cycling process, the intermediate layer should not interfere with lithium dissolution and deposition and must have a smooth surface to guarantee good protective characteristics and strong adhesion with the upper polymer layer.
Additionally, the smooth surface metal layer formed should be capable of reacting (alloying) with the lithium surface. The smooth surface may be obtained by suppressing the alloying when forming the metal layer in a vacuum deposition process.